Ultrasound imaging systems are now frequently used to image sound transmitting materials, such as internal body tissues for medical diagnostic and treatment purposes. Of critical importance to these systems is their ability to produce images of high resolution. Generally, in such systems, radially symmetric (circular) or rectangular apertures are used. In these systems a single aperture is used to both generate and receive acoustic pulses which are constructed into such images. A primary object of the present application is the provision of an apparatus and method using distinct transmitting and receiving apertures which are capable of providing images of superior resolution to those otherwise obtainable using single aperture systems.
In a typical acoustical imaging system, a transducer having a piezoelectric active element is utilized to produce a burst of acoustical energy. This energy is focused by an acoustical lens upon tissue which is located in the focal region. Sound which is scattered by the tissue is then detected by the transducer, and the information thus gained is used to construct a visual display corresponding to the acoustic properties of the subject tissue. Acoustic imaging systems may comprise one or more transducers. When a single transucer is utilized, the transducer is normally pivoted to scan a sector of material to be imaged. Linear transducer arrays have also been used to scan an underlying plane of target material. Linear array systems typically comprise a plurality of discrete rectangular transducers which are sequentially activated (alone or in preselected groupings) to transmit and receive sound to thereby image a series of linear target regions underlying the transducers. If desired, separate sets of these linear array transducers can be used in the send and receive modes to provide a variation in effective aperture size. See for example "New Techniques And Instrumentation In Ultrasonography, pp. 74-76 edited by P.N.T. Wells and M. Ziskin, Churchill Livingstone, N.Y. (1980).
The resolution of acoustic imaging systems is dependent upon a number of different factors. Due in part to diffraction, the intensity of sound in a focal region is at a maximum at the center of the beam, drops laterally to zero at the edge of the Airy disc, and is also peaked at lower intensities in a series of discrete areas surrounding the center of the focal region. When produced by a circular aperture, this central focal region is called the Airy disc, and theoretically contains approximately 84% of the acoustical power of the beam. Surrounding the Airy disc are rings called side-lobes or "Airy rings". Airy rings are rings of decreasing acoustic intensity which extend away from the central focal lobe or Airy disc. Representative plots of the power of acoustic energy in such a focal region are illustrated in FIGS. 1 and 2. The substantial amount of acoustic power distributed in the Airy rings or side-lobes interferes with and limits the resolution of most imaging systems.
Computer simulations and acoustical measurement systems such as ultrasonovision make it possible to calculate, predict, and measure the intensity distribution of acoustic energy expected to be generated and generated by apertures of interest. See "System for Visualizing and Measuring Ultrasonic Wave Fronts", by Mezrich, Etzold, and Vilkomerson, Acoustical Holography, Vol. 6, pp. 165-191 (197); "An Improved System for Visualizing and Measuring Ultrasonic Wave Fronts", by Vilkomerson, et al. Acoustical Holography, Volume 7, ed. Lawrence W. Kessler, Plenum Publishing Corp., (1977) pp. 87-101; and "Ultrasonic Waves: Their Interferometric Measurement and Display", by Mezrich, Vilkomerson and Etzold, Applied Optics, 15:1499 (June 1976). See also "Measuring Pulsed Picometer-Displacement Vibrations by Optical Interferometry", D. Vilkomerson, Applied Physics Letters, 29:3, 183-185 (August, 1976).
The effect on resolution which is caused by diffraction patterns has long been recognized in the fields of optics and acoustics. In both fields circular apertures are generally preferred since the diffraction patterns associated with such apertures are uniform and result in relatively lower diffraction pattern intensities. Other aperture shapes have occasionally been suggested in both fields for specific applications. See Principles In Optics, by N. Born and E. Wolf, Chapter 8, pp. 370-458, (3rd Ed., 1965) Pergamon Press, Oxford, England, which is hereby incorporated by reference. For example, in the field of astronomy, it has been suggested that two closely spaced discrete point light emitters, such as a star and a limb of the sun, can be differentiated by using a square optic aperture which, when properly oriented, exhibits a minimal diffraction pattern intensity in the specific region between the emitters, to thereby permit a better measurement of the apparent distance betweem those emitters. This technique is explained in "SCLERA: An Astronomic Telescope for Experimental Relativity", by J. R. Oleson, C. A. Zanoni, H. A. Hill, A. W. Healy P. B. Clayton, and B. L. Patz, Applied Optics 13:1, (Jan. 1974) 206-211, at 208. Similarly, for specific applications, varying shapes of ultrasound transducers have been suggested. For example, concave, sphercially or parabolically shaped radiators have often been suggested for geometrically focusing ultrasonic beams into a given focal region. Such focused apertures have been suggested as providing large effective depths of field without compromising lateral resolution. It has further been suggested to provide a number of annuli disposed in a flat or concave array for similar purposes. See for example "Electrical Patent Index Profile Booklet", S5 Electromedical, Week D29/032, Oct. 7, 1981, page 102, Derwent Publications Ltd., London WCIX8RP, England. Recently, various analytical expressions used to compute the transient pressure distributions of phased annular arrays with spherical geometry have been disclosed which may lead to improved transducer designs. See "Transient Fields of Concave Annular Arrays", by Arditi, Foster, and Hunt, Ultrasonic Imaging 3:37-61 (1981).
Dual transducer, or dual aperture, ultrasound imaging systems have also been proposed. For example, pulsing and receiving transducers of different resonant frequencies have been utilized for the purpose of investigating frequency changes which may occur within a given target medium. See for example "Electrical Patent Index Booklet", S5 Electromedical, Week D29/032, Oct. 7, 1981 page 97, Derwent Publications Ltd., London WCIX8RP England. In such systems, sound is typically transmitted from the pulsing transducer through a receiving transducer of differential resonant frequency, which inherently acts as a matching layer in this system.
More recently, other dual transducer ultrasound systems have been suggested for use in imaging applications. In "The Conical Scanner: A Two Transducer Ulrtrasound Scatter Imaging Technique", Foster et al, Ultrasonic Imaging 3:62-82 (1981), a system is described comprising a large conical transducer for generating an ultrasound beam that converges into a shapr line focus in the tissue being imaged, and a second circular transducer aimed along the axis of the cone to detect scattered ultrasound as a function of time, to thereby gather information which may be converted into a high resolution image.
It has also been suggested to use multiple apertures of differential focal lengths for alternatively imaging selected tissue portions. By using different sized apertures, different depths of field can be utilized to image these tissues portions. See U.S. Pat. No. 4,168,628 (Vilkomerson).
While the above-described acoustic imaging systems have experienced some degree of success, each of these systems has utilized transducer(s) which are radially symmetric with respect to the focal axis, and therefore produce substantially symmetric side-lobe patterns. While it has been recognized that the side-lobe echoes detected by such systems have interferred with image resolution, such limitations have heretofore been considered to be inherent drawbacks of acoustic imaging systems.